We have determined the mean gas column density of the knots from a radiative transfer analysis of the CO 2–1 line emission, using the code MadeX [1]} in the Large Velocity Gradient approximation and the CO-H\({}_2\)  collisional coefficients from [2]}. In this simple model, we assume that the CO emission measured towards the knots actually arises from a single physical component. In other words, the contributions of the entrained gas envelope and the inner jet material along the line of sight, both discussed in Sect. 4.3, are neglected with respect to that of the knots. Linewidths and intensities were obtained from a Gaussian fit to the line profiles at the knot flux peak (see Table REF ). The knot peak intensity was sometimes found to lie off the jet main axis by a fraction of a beam. This indirectly reveals a complex knot structure at subarcsec scale, with some small velocity shifts between the intensity peak and the jet main axis.
We have adopted the jet H\({}_2\)  density and temperature derived by [3]} and [4]} in the southern and northern lobes from a CO multi-transition analysis.
The total H\({}_2\)  mass and column density were subsequently obtained from N(CO) adopting a standard CO to H\({}_2\)  relative abundance ratio of \(10^{-4}\) .
